Spanish fly (Lytta vesicatoria) is an emerald-green beetle in the blister beetle family (Meloidae). It and other such species were used in preparations offered by traditional apothecaries, often referred to as Cantharides or Spanish fly. The insect is the source of the terpenoidcantharidin, a toxic blistering agent once used as an aphrodisiac.

L. vesicatoria is sometimes called Cantharis vesicatoria,[1] although the genus Cantharis is in an unrelated family, Cantharidae, the soldier beetles.[2]

The Spanish fly is a mainly southern European species[10][11] although its range of habitats is more completely described as being "throughout southern Europe and eastward to Central Asia and Siberia,"[3] alternatively as being throughout Europe, and parts of northern and southern Asia (excluding China).[12] It occurs locally in southern Great Britain[13] and Poland.[14]

The defensive chemical cantharidin, for which the beetle is known, is produced only by males; females obtain it from males during mating, as the spermatophore contains some. This may be a nuptial gift, increasing the value of mating to the female, and thus increasing the male's reproductive fitness.[16]

The female lays her fertilised eggs on the ground, near the nest of a ground-nesting solitary bee. The larvae are very active as soon as they hatch. They climb a flowering plant and await the arrival of a solitary bee. They hook themselves on to the bee using the three claws on their legs that give the first instar larvae their name, triungulins (from Latin tri, three, and ungulus, claw). The bee carries the larvae back to its nest, where they feed on bee larvae and the bees' food supplies. The larvae are thus somewhere between predators and parasites. The active larvae moult into very different, more typically scarabaeoid larvae for the remaining two or more instars, in a development type called hypermetamorphosis. The adults emerge from the bees' nest and fly to the woody plants on which they feed.[17][3]

Cantharidin, the principal active component in preparations of Spanish fly, was first isolated and named in 1810 by the French chemist Pierre Robiquet, who demonstrated that it was the principal agent responsible for the aggressively blistering properties of this insect's egg coating. It was asserted at that time that it was as toxic as the most violent poisons then known, such as strychnine.[18]

The active agent has been estimated present at about 0.2-0.7 mg per beetle, males producing significantly more than females. The beetle secretes the agent orally, and exudes it from its joints as a milky fluid. The potency of the insect species as a vesicant has been known since antiquity and the activity has been used in various ways. This has led to its small-scale commercial preparation and sale, in a powdered form known as cantharides (from the plural of Greek κανθαρίς, Kantharis, beetle), obtained from dried and ground beetles. The crushed powder is of yellow-brown to brown-olive color with iridescent reflections, is of disagreeable scent, and is bitter to taste. Cantharidin, the active agent, is a terpenoid, and is produced by some other insects, such as Epicauta immaculata.[4][1][19]

Preparations from L. vesicatoria and its active agent have been implicated in both inadvertent[19] and intentional poisonings.[19] Froberg notes a 1954 manslaughter case where cantharidin was administered in a coconut-flavoured candy as an intended aphrodisiac, resulting in illness and eventual death of two women (agent identified postmortem), and in facial blistering and criminal conviction of the perpetrator.[19]

In Morocco and other parts of North Africa, spice blends known as ras el hanout sometimes included as a minor ingredient "green metallic beetles", inferred to be cantharides from L. vesicatoria, although sale of this in Moroccan spice markets was banned in the 1990s.[25]Dawamesk, a spread or jam made in North Africa and containing hashish, almond paste, pistachio nuts, sugar, orange or tamarind peel, cloves, and other various spices, occasionally included cantharides.[26]

The Venezuelan leader Simón Bolívar may have been accidentally poisoned by application of Spanish fly.[28]

Arthur Kendrick Ford was convicted and given a multiyear prison sentence in 1954 for the unintended deaths of two women surreptitiously given candies laced with cantharidin, which were intended to act as an aphrodisiac.[19]